Gender differences have been observed in pain and opioid sensitivity. In general, females have lower nociceptive thresholds and achieve less analgesia from morphine-like (mu) opioids. During different reproductive stages female rats experience different pain thresholds, suggesting that the hormonal environment may modulate their nociceptive responses. The proposed studies will examine pain and opioid sensitivity in ovariectomized (OVX) rats with and without steroid replacement Behavior and gene expression will be concurrently examined. We will measure not only the response to an acute thermal stimulus (tail flick) but also models of subacute (formalin and intrathecal, IT NMDA) and chronic (nerve constriction) nociception and a test for mechanical sensitivity. Because the glutamatergic receptor system is an important modulator of pain sensitivity in animals and humans and in the development of morphine tolerance, we will examine this system together with the opioid system. Steroids can have powerful effects on gene expression and therefore we will measure their effects on glutamate and opioid receptors and neuropeptide mRNA and protein levels. The project will be conducted as an integrated mentoring and collaborative relationship between investigators at Hunter College of the City University of New York and Weill Medical College of Cornell University of New York and Weill Medical College of Cornell University. The Quinones' group will focus on understanding the role of steroids in the modulation of pain (nociception-tail flick and formalin tests) and opioid (mu and kappa) sensitivity (acute, development of tolerance and efficacy of NMDA receptor antagonists). The Inturrisi group will train Quinones' group in, not only the techniques necessary to produce the pain and opioid tolerance models, but also in the design and analysis (e.g., dose-response) of the proposed studies. The mentoring experience will extend to Hunter College students, who will have the opportunity to obtain further training in this area of neuroscience resulting in the expansion of neuroscience research at Hunter. The research expertise acquired through the mentoring-collaborative interactions will ensure that Dr. Quinones is more competitive in obtaining future independent funding. Research into the mechanisms underlying gender differences in pain and opioid sensitivity is of considerable interest since it can reveal systems, e.g., hormonal which modulate pain and opioid responsiveness, and therefore can provide a new understanding of how females cope with the aversive stimuli that may accompany copulation, parturition, and nursing. The proposed studies may also provide new insights into pain management approaches for females including those utilizing estrogen or progesterone based contraceptives or estrogen replacement treatment after menopause.